The program counter is incremented during the FETCH phase, before the instruction has even been decoded. Why increment it so early rather than after the instruction completes?

A. Because the address has already been used, so the PC is free for the next one.

Once the fetch has used the address, advancing the PC immediately leaves the machine pointing at the default next instruction. That default is what makes branching clean, a jump is one register assignment, not special sequencing logic. It also means the PC already holds the next address during execution, which is where relative branch offsets are measured from.